The present invention relates to a light scattering film, and a liquid crystal display device having a light scattering film. In the present invention, the terms xe2x80x9cscatteringxe2x80x9d and xe2x80x9cdiffusionxe2x80x9d are used with no distinction as long as they are used in relation to optics.
A light scattering film is used in a liquid crystal display device in order to achieve a wide viewing angle or to achieve display of a uniform brightness over the entire screen. In a reflection type liquid crystal display device, such a light scattering film is pasted to a display surface of a liquid crystal panel.
It was customary in the past to use a light scattering film prepared by roughening the surface of a polymer film. Such a light scattering film is prepared by applying a physical processing such as a sand blasting treatment to the surface of a polymer film or by a chemical treatment using an acidic or basic solution.
The light scattering film permits all the incident light to be diffused without relying on the incident angle, the incident direction or the incidence plane. Therefore, where such a light scattering film is used in a reflection type liquid crystal display device utilizing the solar light or external illumination as a light source, the light scattering takes place both at the time when the light emitted from the light source is incident on the liquid crystal panel and at the time when the light modulated by the liquid crystal layer became reflected and emerges out of the liquid crystal panel. As a result, a double image is observed, or the image is blurred. In other words, the displayed image is made unclear or the contrast is lowered.
Since the light scattering film diffuses the incident light isotropically, a wide viewing angle can be obtained. However, the viewing angle and the brightness of display bears a trade-off relationship. Specifically, it is difficult to achieve a bright display by using the light scattering film.
In general, it suffices for the liquid crystal display device used in a portable telephone or PDA to have a viewing angle permitting a single user to recognize the displayed image. In other words, the viewing angle in a horizontal direction need not be unduly large in the liquid crystal display device used in these instruments. Since these instruments are used at a height of the eye or disposed on the lap, the viewing angle should desirably be large in a vertical direction. By contraries, in a liquid crystal television, etc., the viewing angle of the liquid crystal display device need not be large in a vertical direction and should desirably be large in a horizontal direction.
As describe above, the required direction of a large viewing angle differs depending on the use of the liquid crystal display device. Therefore, a sufficient brightness of display and a large viewing angle can be obtained simultaneously by increasing the viewing angle in a required direction alone. It follows that the light scattering film used in a liquid crystal display device is required to scatter light anisotropically.
It is known to the art that a polymer film having light diffusing fine particles dispersed therein can be used as a light scattering film. In order to realize the above-noted light scattering characteristics in the light scattering film, various efforts are being made in an attempt to control the refractive index, size, shape, etc. of the light diffusing fine particles. However, it is technically difficult to realize the particular light scattering characteristics by the method referred to above. Even if realized, the light scattering characteristics are not satisfactory enough to put the light scattering film to practical use.
For example, Japanese Patent Disclosure (Kokai) No. 8-201802 discloses a light scattering plate which permits transmitting the light incident on one main surface while suppressing the light scattering substantially completely and also permits selectively scattering the light incident on the other main surface. However, this prior art simply teaches that the light scattering plate is obtained by solidifying transparent fine particles with a transparent polymerizable high molecular compound, failing to teach the specific construction of the light scattering plate. It is considered reasonable to understand that the light scattering plate disclosed in JP ""802 is equal in construction to the known polymer film having fine light diffusing particles dispersed therein, and therefore, the light scattering characteristics above cannot be obtained.
Japanese Patent Disclosure No. 9-152602 discloses a liquid crystal display device using a hologram. The device disclosed in this prior art is a transmitting type liquid crystal display device having a back light as a light source arranged behind a liquid crystal panel. In this device, a light scattering plate is arranged between the liquid crystal panel and the back light, and a hologram is arranged on the front face of the liquid crystal panel. The hologram thus arranged permits anisotropically diffusing the light. In the case of using a hologram, however, the light is unavoidably dispersed to exhibit the spectral colors, with the result that the color of the displayed image differs depending on the viewing point.
An object of the present invention is to provide a light scattering film which permits displaying a clear image when used in a liquid crystal display device and a liquid crystal display device using the particular light scattering film.
Another object is to provide a light scattering film which permits realizing a sufficiently large viewing angle and also permits a bright display when used in a liquid crystal display device and a liquid crystal display device using the particular light scattering film.
Still another object of the present invention is to provide a light scattering film which permits preventing the color of the displayed image from being changed in accordance with movement of the viewing point and a liquid crystal display device using the particular light scattering film.
According to an aspect of the present invention, there is provided a light scattering film, comprising a plurality of first transparent regions each having a fibril-like cross section, and a plurality of second transparent regions differing in refractive index range from the first transparent region, wherein each of the first transparent regions is positioned to permit the long axis of the fibril-like cross section to cross one main surface of the film and to permit each of the fibril-like cross sections of the first transparent regions to be sandwiched between adjacent second transparent regions in a direction of the short axis of the fibril-like cross section.
According to another aspect of the present invention, there is provided a liquid crystal display device, comprising a liquid crystal panel, and a light scattering film mounted on one main surface of the liquid crystal panel, wherein the light scattering film includes a plurality of first transparent regions each having a fibril-like cross section, and a plurality of second transparent regions differing in refractive index range from the first transparent region, and wherein each of the first transparent regions is positioned to permit the long axis of the fibril-like cross section to cross one main surface of the film and to permit each of the fibril-like cross sections of the first transparent regions to be sandwiched between adjacent second transparent regions in a direction of the short axis of the fibril-like cross section.
The light scattering film of the present invention comprises a plurality of first transparent regions each having a fibril-like cross section and a plurality of second transparent regions each interposed between adjacent first transparent regions. The first transparent region and the second transparent region differ from each other in refractive index range. It follows that, where each of the fibril-like cross sections of the first transparent regions has a width narrow enough to diffuse the light incident in a direction nearly parallel to the long axis of the fibril-like cross section, the light scattering film of the present invention permits the light incident in a direction of the long axis of the fibril-like cross section of the first transparent region to be scattered and also permits the light incident in a direction of the short axis of the fibril-like cross section to be transmitted without being scattered. In other words, the light scattering film of the present invention exhibits various light scattering characteristics dependent on the incident direction of the light.
Where the light scattering film of the present invention is used in a reflection type liquid crystal display device, the light emitted from a light source such as the sun or an external illumination is scattered when the light is incident on the liquid crystal panel, and the light emerging out of the liquid crystal panel toward the observer is transmitted without being scattered. It follows that a reflection type liquid crystal display device using the light scattering film of the present invention permits displaying a clear image.
Where the light scattering film of the present invention is used in a transmitting type liquid crystal display device, the effective light for the display can be selectively scattered, leading to a high display contrast.
Further, where the light scattering film of the present invention is used in a transmitting type liquid crystal display device, the light emitted from a light source such as a back light is partly scattered, with the remainder being transmitted without being scattered or being subjected to total reflection. Where the light scattering film is designed to allow almost all the light components effective for the display to be scattered, it is substantially impossible for the transmitted light that is not scattered to be perceived by the observer. On the other hand, the light reflected in a manner of total reflection, which is not scattered, is utilized again for the display so as to suppress the loss of light, making it possible to achieve a bright display. Incidentally, the light scattering film of the present invention may be arranged either between the liquid crystal panel and the back light or on the observer""s side of the liquid crystal panel.
In the light scattering film of the present invention, it is desirable for those portions of the first and second transparent regions which are exposed to a main surface of the film to have a shape elongated in substantially one direction. In this case, it is possible to impart anisotropy to the light scattering direction. What should be noted is that, since the light diffusing direction can be controlled, it is possible to realize a large viewing angle and a sufficiently bright display simultaneously in the case of using the light scattering film of the present invention in a liquid crystal display device.
The light scattering film of the present invention does not split light, and there is not color dispersion to produce a spectral distribution unlike the hologram, with the result that the color of the displayed image is not changed in accordance with the movement of the viewing point.
In the present invention, the long axes of the fibril-like cross sections of the first transparent regions are substantially parallel to each other. Each of the second transparent regions also has a fibril-like cross section. It is desirable for the long axis of the fibril-like cross section of each of the first transparent regions to be substantially parallel to the long axis of the fibril-like cross section of each of the second transparent regions. In this case, if the fibril-like cross section of each of the second transparent regions has a width narrow enough to diffuse the light incident in a direction parallel to the long axis of the fibril-like cross section, each of the first and second transparent regions contributes to the scattering of light, leading to a high scattering performance.
Each of the first and second transparent regions, which may be shaped in various fashions such as a needle-like shape, a columnar shape or a ramentum-like shape, are generally formed to have fibril-like shape. It is possible for each of the first and second transparent regions to extend from one main surface to the other main surface of the light scattering film. It is also possible for at least a part of the first and second transparent regions to have a laminate structure laminated one upon the other along the long axis of the fibril-like cross section. Further, those portions of the first and second transparent regions which are exposed to one main surface of the light scattering film may be arranged as a speckle pattern. The particular construction can be formed by utilizing, for example, a speckle pattern.
Where the long axis of the fibril-like cross section of each of the first and second transparent regions is substantially perpendicular to one main surface of the light scattering film, the light scattering film of the present invention can be used effectively in a transmitting type liquid crystal display device. On the other hand, where the long axis of the fibril-like cross section of each of the first and second transparent regions is inclined relative to one main surface of the light scattering film, the light scattering film of the present invention can be used effectively in a reflection type liquid crystal display device.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part they will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.